All U.S. patents cited below are herein entirely incorporated by reference.
As used herein, the term “thermoplastic” is intended to mean a polymeric material that will melt upon exposure to sufficient heat but will retain its solidified state, but not prior shape without use of a mold or like article, upon sufficient cooling. Specifically, as well, such a term is intended solely to encompass polymers meeting such a broad definition that also exhibit either crystalline or semi-crystalline morphology upon cooling after melt-formation through the use of the aforementioned mold or like article. Particular types of polymers contemplated within such a definition include, without limitation, polyolefins (such as polyethylene, polypropylene, polybutylene, and any combination thereof), polyamides (such as nylon), polyurethanes, polyester (such as polyethylene terephthalate), and the like (as well as any combinations thereof).
Thermoplastics have been utilized in a variety of end-use applications, including storage containers, medical devices, food packages, plastic tubes and pipes, shelving units, and the like. Such base compositions, however, must exhibit certain physical characteristics in order to permit widespread use. Specifically within polyolefins, for example, uniformity in arrangement of crystals upon crystallization is a necessity to provide an effective, durable, and versatile polyolefin article. In order to achieve such desirable physical properties, it has been known that certain compounds and compositions provide nucleation sites for polyolefin crystal growth during molding or fabrication. Generally, compositions containing such nucleating compounds crystallize at a much faster rate than un-nucleated polyolefin. Such crystallization at higher temperatures results in reduced fabrication cycle times and a variety of improvements in physical properties, such as, in one example, stiffness.
Such compounds and compositions that provide faster and or higher polymer crystallization temperatures are thus popularly known as nucleators. Such compounds are, as their name suggests, utilized to provide nucleation sites for crystal growth during cooling of a thermoplastic molten formulation. Generally, the presence of such nucleation sites results in a larger number of smaller crystals. As a result of the smaller crystals formed therein, clarification of the target thermoplastic may also be achieved, although excellent clarity is not always a result. The more uniform, and preferably smaller, the crystal size, the less light is scattered. In such a manner, the clarity of the thermoplastic article itself can be improved. Thus, thermoplastic nucleator compounds are very important to the thermoplastic industry in order to provide improved clarity, physical properties and/or faster processing.
Dibenzylidene sorbitol compounds are common nucleator compounds, particularly for polypropylene end products. Compounds such as 1,3-O-2,4-bis(3,4-dimethylbenzylidene) sorbitol (hereinafter DMDBS), available from Milliken Chemical under the trade name Millad® 3988, provide excellent nucleation characteristics for target polypropylenes and other polyolefins. Other well known compounds include sodium benzoate, sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate (from Asahi Denka Kogyo K.K., known as NA-11), talc, and the like.
Other acetals of sorbitol and xylitol are typical nucleators for polyolefins and other thermoplastics as well. Dibenzylidene sorbitol (DBS) was first disclosed in U.S. Pat. No. 4,016,118 by Hamada, et al. as an effective nucleating and clarifying agents for polyolefins. Since then, large number of acetals of sorbitol and xylitol have been disclosed. Representative US patents include: Kawai, et al., U.S. Pat. No. 4,314,039 on di(alkylbenzylidene) sorbitols; Mahaffey, Jr., U.S. Pat. No. 4,371,645 on di-acetals of sorbitol having at least one chlorine or bromine substituent; Kobayashi, et al., U.S. Pat. No. 4,532,280 on di(methyl or ethyl substituted benzylidene) sorbitol; Rekers, U.S. Pat. No. 5,049,605 on bis(3,4-dialkylbenzylidene) sorbitols including substituents forming a carbocyclic ring.
Other examples of effective nucleating agents are the metal salts of organic acids. Wijga in U.S. Pat. Nos. 3,207,735, 3,207,736, and 3,207,738, and Wales in U.S. Pat. Nos. 3,207,737 and 3,207,739, all patented Sep. 21, 1966, suggest that aliphatic, cycloaliphatic, and aromatic carboxylic, dicarboxylic or higher polycarboxylic acids, corresponding anhydrides and metal salts are effective nucleating agents for polyolefin. They further state that benzoic acid type compounds, in particular sodium benzoate, are the best embodiment of the nucleating agents.
Another class of nucleating agents, alluded to above, was suggested by Nakahara, et al. in U.S. Pat. No. 4,463,113, in which cyclic bis-phenol phosphates were disclosed as nucleating and clarifying agents for polyolefin resins. Kimura, et al. then suggests in U.S. Pat. No. 5,342,868 that the addition of an alkali metal carboxylate to basic polyvalent metal salt of cyclic organophosphoric ester can further improve the clarification effects of such additives. This patent thus at best discloses the reaction of a salt with a base, although such a reaction is never actually stated to occur within this reference. Compounds that are based upon these technologies are marketed under the trade name NA-11 and NA-21.
U.S. Pat. Nos. 5,300,549 and 5,319,012, as well as published PCT Application WO 01/53404 all disclose the addition of metal salts to acidic catalyst residues (i.e., aluminum catalyst by-products having low pH levels, but no acid functionalities, such as carboxyl groups, thereon) already present within a target resin as a means to provide nucleation and clarity to thermoplastics. None of these patents discloses the ability to introduce separately to molten resins different soluble compounds therein to generate a bicyclic or monocycloaliphatic nucleating agent in situ.
U.S. Pat. No. 5,231,126 teaches beta-nucleation of polypropylene through the reaction of linear aliphatic or aromatic acids with oxides, hydroxides, or salts within target polypropylene. Such a method requires specific types of dibasic acids to accomplish the desired beta-polypropylene crystal generation at sufficiently high levels. Generally, however, the production of such high levels of beta-crystals is unwanted and thus such beta-nucleation of target polypropylene is to be avoided. In fact, in order to provide the best clarity in target polypropylene, the amount of beta-crystals should be at most 5% of the total amount of polypropylene crystals within the target plastic itself, with the closer to 0% beta-crystals the better for the best level of low haze (and thus a non-opaque result).
Furthermore, a certain class of bicyclic compounds, such as bicyclic dicarboxylic acid and salts, have been taught as polyolefin nucleating agents as well within Patent Cooperation Treaty Application WO 98/29494, to Minnesota Mining and Manufacturing. The best working example of this technology is embodied in disodium bicyclo[2.2.1]-5-heptene-2,3-dicarboxylate and formulations with such compounds.
Such compounds all impart relatively high polyolefin crystallization temperatures; however, each also exhibits its own drawback for large-scale industrial applications.
For example, of great interest is the compatibility of such compounds with different additives widely used within typical polyolefin (e.g., polypropylene, polyethylene, and the like) plastic articles. For instance, calcium stearate is a very popular acid neutralizer present within typical polypropylene formulations to protect the end product from catalyst residue attack. Unfortunately, most of the nucleator compounds noted above exhibit deleterious reactions with such compounds within polyolefin articles. For sodium, and other like metal ions, it appears that the calcium ion from the stearate transfers positions with the sodium ions of the nucleating agents, rendering the nucleating agents ineffective for their intended function. As a result, such compounds sometimes exhibit unwanted plate-out characteristics and overall reduced nucleation performance (as measured, for example) by a decrease in crystallization temperature during and after polyolefin processing. Other processing problems are evident with such compounds as well.
As noted above, beta-crystal polypropylene structures are preferably avoided, with at most 5% in terms of total numbers of polypropylene crystals in the final article preferred. Such beta-crystals do generally aid with impact resistance and stiffness of the target polypropylene article; however, in terms of clarity, the presence of above 5% is detrimental and such a small amount can cause high degrees of unwanted opacity in target transparent resin articles.
Other problems encountered with the standard nucleators noted above include inconsistent nucleation due to dispersion problems, resulting in stiffness and impact variation in the polyolefin article. Substantial uniformity in polyolefin production is highly desirable because it results in relatively uniform finished polyolefin articles. If the resultant article does not contain a well-dispersed nucleating agent, the entire article itself may suffer from a lack of rigidity and low impact strength. Thus, simplicity of production as well as effective dispersion of the initial components (reactants) within the target thermoplastic resin is needed, as well as effective nucleation thereby.
Furthermore, storage stability of nucleator compounds and compositions is another potential problem with thermoplastic nucleators and thus is of enormous importance as well. Since nucleator compounds are generally provided in powder or granular form to the polyolefin manufacturer, and since uniform small particles of nucleating agents is imperative to provide the requisite uniform dispersion and performance, such compounds must remain as small particles through storage. Certain nucleators, such as sodium benzoate, exhibit high degrees of hygroscopicity such that the powders made therefrom hydrate easily resulting in particulate agglomeration. Such agglomerated particles may require further milling or other processing for deagglomeration in order to achieve the desired uniform dispersion within the target thermoplastic. Furthermore, such unwanted agglomeration due to hydration may also cause feeding and/or handling problems for the user.
Also of great interest is the compatibility of such compounds with different additives widely used within typical polyolefin (e.g., polypropylene, polyethylene, and the like) plastic articles. As noted previously, calcium stearate compatibility is particularly important. Unfortunately, most of the nucleators compounds noted above (such as sodium benzoate, NA-11, disodium bicyclo[2.2.1] heptene dicarboxylate) exhibit much deleterious nucleating efficacy with such compounds within polyolefin articles. In order to avoid combinations of these standard nucleators and calcium salts, other nonionic acid neutralizers, such as dihydrotalcite (DHT4-A), would be necessary for use in conjunction with such nucleators. Such a combination, however, has proven problematic in certain circumstances due to worsened aesthetic characteristics (e.g., higher haze), and certainly higher costs in comparison with standard calcium salts.
Some nucleating agents, such as certain DBS derivatives, exhibit certain practical deficiencies such as a tendency to plate-out at high processing temperatures. DBS derivatives, particularly where the aromatic rings are mono-substituted, show much improved thermal stability. However, such compounds also tend to exhibit undesirable migratory properties coupled with problematic organoleptic deficiencies within certain polyolefin articles. As a result, such compounds are limited in their practical in some important areas, such as medical device packaging.
These noticeable problems have thus created a long-felt need in the polyolefin nucleator compound industry to provide such compounds that do not exhibit the aforementioned problems and provide excellent peak crystallization temperatures for the target polyolefin themselves. To date, the best compounds for this purpose remain those noted above. Unfortunately, nucleators exhibiting exceptionally high peak crystallization temperatures, low hygroscopicity, excellent thermal stability, and non-migratory properties within certain target polyolefin, and compatibility with most standard polyolefin additives (such as, most importantly, calcium stearate) have not been developed within the polyolefin nucleator industry.
Furthermore, methods of utilizing the benefits of typical salt additives within thermoplastics and polyolefins have heretofore been unexplored. In such a potential scenario, the ability to react added salts, such as acid scavengers, with nucleator precursor compounds could possibly provide, in situ, the desired type and amount of ultimate nucleating agent within the target thermoplastic resin. To date there has been no teaching of the viability of providing nucleation of thermoplastics (preferably polyolefins, more preferably polypropylene of very low beta-crystal levels) through the reaction of added bicyclic or monocycloaliphatic components (preferably, though not necessarily acids of such types) with added organic acids. Again, the closest art discloses either the production of high beta-crystal polypropylene levels, reactions between organic salts and bases, or the introduction of organic salts to already-present catalyst residues within target thermoplastics. Such an in situ reaction between added components could potentially reduce cost through supplying the needed metal ions from the already-present salts, as well as through simplification of the overall reaction itself through improved dispersion of the initially added compounds. Such a situation could be beneficial, for example, if the nucleator precursor exhibits better stability or other characteristic prior to introduction within target thermoplastic, as compared with the desired ultimate nucleating agent. However, again, no such teaching or fair suggestion has been presented within the pertinent prior art of this specific potential reaction.